I refill.  Unless you prechill the bottle in the freezer, you will
not be able to get even close to the amount into the tank that came
out, and therefore will not be able to overfill it, AFAIK.  When I do
prechill, I preweigh the empty bottle and write the empty weight on
the bottle.  Then When I fill it, I weight it to make sure I don't
overfill.  You get a feel for when to stop filling, but I still check
it.  I've had one that leaked so far.  I just hooked to my stove and
used it like regular, then threw it away empty.  I just use the
refilled tanks around home and probably would not carry them inside
the van.  I like the suggestion people made of the little pancake
tanks.  How much do they hold?

Edward